Luminaires which provide a generally horizontal distribution of emitted light have long been employed for marking objects of navigational interest, such as buoys and fixed obstructions. The horizontal distribution of light increases the efficiency of such luminaires, as it causes the light generated to be concentrated in the generally horizontal region where viewers are most likely to be located. This desired horizontal distribution has classically been attained by using cylindrical Fresnel lenses surrounding a centrally-located lamp, such as taught in U.S. Pat. No. 6,099,148. While such lenses can concentrate a large portion of light emitted from a lamp into a substantially horizontal distribution, those portions of the light which are directed substantially vertically upward or downward will not enter such lenses, and thus are wasted. To avoid the loss of these substantially vertically-directed portions of the light, U.S. Pat. Nos. 5,230,560 and 5,335,157 teach the use of reflectors to redirect these portions of the light in a substantially horizontal direction in order to provide greater efficiency for the luminaire.
More recently, there has been a need to redirect light from LED's so as to provide a distribution of light suitable for illuminating a backlit display. U.S. Pat. Nos. 6,598,998; 6,607,286; and 6,679,621 teach side-emitting lenses that redirect the light emitted by LED's to obtain a distribution of light in a direction substantially perpendicular to an axis of symmetry of the lens; this distribution is intended to increase efficiency by directing most of the light generated into a thin backlighting panel light guide, into which the lens is inserted, and minimizing the portion of light not directed into the panel. These lenses employ an internal reflecting surface which is generally conical in overall shape, and which functions to reflect light from the LED to an associated refracting surface, which in turn refracts the light to exit the lens in a substantially perpendicular direction, so as to enter the backlighting panel. This refracting surface can be a sawtooth surface, somewhat resembling a portion of a cylindrical Fresnel lens, or can be formed by a simple frustoconical surface which forms an oblique angle to the axis of symmetry of the lens. The lens has a cavity in which the LED is located, with the reflecting surface located above the cavity. In some embodiments, the lens has a second refracting surface, which forms a smooth convex curve designed to further refract the light so as to direct it into the backlighting panel.
U.S. Pat. No. 7,153,002 states that the side-emitting lenses for LED's taught in the '998, '286, and '621 patents are limited by the size of LED light source which can be accommodated, as some light is not directed out the sides when larger LED light sources are employed. The '002 patent teaches a lens structure which is stated to be more suitable for use with larger LED light sources, such as those where multiple LED's are employed to provide greater light intensity or where the white light is formed by combining the light from three primary-color LED's. In the '002 lens structure, the lens has both an upper reflecting surface and an inner reflecting surface, these reflecting surfaces serving to direct the light to be emitted through side surfaces.
Since luminaires for marking obstructions are frequently employed at remote locations, such as on buoys, it is desirable for such luminaires to employ LED's to generate light. LED's provide advantages of reliability and relatively low power consumption, reducing the size of batteries needed for power. While the side-emitting lenses discussed above apparently serve well for backlighting applications, they have not been found practical for use in a luminaire for marking obstructions. Such lenses are designed only to assure that the light emitted through the side surfaces will enter a light guide of a backlighting panel, and this distribution of light has not been found to be sufficiently well-directed to be practical in providing the needed intensity of light for a marker luminaire without undue power consumption.